Wastewater treatment plants as pathways of microlitter to the aquatic environment

Microlitter and its synthetic sub-type microplastics, defined as anthropogenic particles less than 5 mm in size, are an ever-increasing form of marine litter that has received considerable attention over the past decades. Among the numerous possible sources and pathways that allow this microlitter to enter aquatic environments, wastewater treatment plants (WWTPs) have been suggested as one of the more significant pathways. This thesis examines the step-wise removal of microlitter during the purification process within conventional WWTPs utilizing conventional activated sludge processes. In addition, the removal of microplastics from wastewater effluents using five different advanced final-stage treatment technologies was investigated and the microplastic discharge from existing Finnish WWTPs into the aquatic environments was assessed. To further evaluate the environmental risk that microlitter may pose to aquatic food webs, their ingestion by Baltic blue mussels (Mytilus trossulus) was studied at a wastewater receiving area in the Gulf of Finland in the Northern Baltic Sea. These results show that conventional wastewater treatment using primary and secondary treatments can efficiently remove (> 99 %) microlitter arriving at the WWTP in influent. Most (98%) of the microlitter was removed during primary treatment. The activated sludge process further decreased (~ 88%) the microlitter concentration. During the wastewater treatment, most of the microlitter (> 99.5 %) was retained in the raw and excess sludge. However, part (~ 20 %) of the retained microlitter was recycled back to the treatment process along with the reject water. The removal of microplastics can be further enhanced by advanced final-stage wastewater treatment technologies. Membrane bioreactor removed an additional 99.9% of microplastics during treatment. Sand filtration removed 97%, dissolved air flotation removed 95 %, and disc filtration removed 40 – 98.5% of the MPs. Biologically active filtration did not have any impact on the microplastic concentration. According to our estimations, Finnish WWTPs annually discharge ~ 480 billion microplastic particles into aquatic environments. As vast volumes of wastewaters are constantly discharged into the aquatic environments, the role of WWTPs as pathways may be significant. However, to further evaluate the relative importance of the role of WWTPs as pathway for microplastics, information on other pathways are also needed. The results from the wastewater receiving environment indicate that the WWTPs may influence the microlitter and microplastics abundance and composition detected in biota. Blue mussels collected from the wastewater receiving area had higher microlitter content than those from the reference site.